The present invention concerns the field of energy storage systems and relates to a subclass of cathode materials which are used as intercalation materials in lithium ion cells which are a main component of lithium ion batteries. A lithium ion cell is generally defined as an electrochemical element in which lithium ions occur as ionic charge carriers. The typical demands made of an energy store at the battery level, in particular, in respect of its energy density, power density, safety, longevity, environmental friendliness and costs, also apply at the cell level and ultimately for the individual components, i.e., also for cathode materials.
The present invention is based on the main class of the LiMn2O4 spinels, in particular, the LiMe1vMn1.5-vO4 where Me1={Cr, Fe, Co, Ni, Cu, Zn}, 0.3≦v≦0.7, as described in T. Ohzuku et al., Journal of Power Sources 81-82, pp. 90-94 (1999) as high-voltage spinels. In contrast to other classes of cathode materials, especially LiCoO2, LiNixMnyCozO2, LiNixCoyAlzO2, this main class is, according to Arnold, G., Journal of Power Sources 119-121, pp. 247-251 (2003), known for a high power density combined with improved intrinsic safety. Further advantages are its environmental friendliness and the somewhat lower materials costs due to the absence of cobalt. The high-voltage spinel LiNi0.5Mn1.5O4 has been the subject of intensive research during the last decade. A distinction is made between the ordered spinels having the space group P4332 and the disordered spinels (Fd-3m), which, according to Yang, T., Journal of Alloys and Compounds 502, pp. 215-219 (2010) and Kunduraci, M., Chemistry of Materials 18, pp. 3585-3592 (2006), are more suitable as a cathode material because of improved electronic conductivity. Doping usually stabilizes the disordered spinel and partly or completely eliminates the LixNi1-xO foreign phase which frequently occurs in the synthesis.
R. Santhanam, B. Rambabu, Journal of Power Sources 195 pp. 5442-5451 (2010), G. Liu, L. Wen, Y. Liu, Journal of Solid State Electrochemistry 14, pp. 2191-2202 (2010), and T.-F. Yi, Y. Xie, M.-F. Ye, L.-J. Jiang, R.-S. Zhu, Y.-R. Zhu, Ionics 17, pp. 383-389 (2011) describe the high-voltage spinel LiNi0.5Mn1.5O4, in particular, the influence of doping. On the cation side, nickel and manganese are partly replaced by magnesium, chromium, cobalt, iron, titanium, iron-titanium, copper, aluminum, zirconium and ruthenium, while on the anode side, oxygen is replaced by fluorine and sulfur. Many positive influences are ascribed to dopings as long as only small amounts, usually x≦0.15, are added, although different synthesis processes, morphologies, characterization methods and cell preparations make direct comparison of the influences of doping more difficult. The effects of doping in the high-voltage range, as described therein, are a slightly increased redox potential (Mg2+, Ti4+), reduced polarization (Ru4+, F−), improved cycling stability (Mg2+, Cr3+, Fe3+, F−), improved electronic conductivity (Mg2+, Cr3+, Fe3+, Co3+, Ru4+), improved Li+ ion conductivity (Ti4+, Co3+, Ru4+), improved performance as a cathode material (Cr3+, Fe3+, Ru4+, F−), a greater binding energy to oxygen (Cr3+, Fe3+, Co3+), improved structural stability (Cr3+), improved heat resistance (Cr3+, F−), and improved resistance to the standard electrolyte (Cr3+, Fe3+, F−). Fluorine here increases the resistance to hydrogen fluoride, while Cr3+ and Fe3+ are stated in Goodenough, J. B. et al., Journal of Power Sources 196, pp. 6688-6694 (2011) to reduce electrolyte oxidation at voltages above 4.5 V by formation of a passivating covering layer on the interface of the cathode material to the electrolyte (solid electrolyte interphase, known as SEI layer for short). Apart from F− doping, a greater stability to hydrogen fluoride, which can form in the standard electrolyte system, can, according to G. Liu, L. Wen, Y. Liu, Journal of Solid State Electrochemistry 14, pp. 2191-2202 (2010), be achieved by means of coatings.
The electrochemical results in respect of the influence of doping consist virtually exclusively of cycling in the high-voltage range, i.e., from 3.5 V to 5.0 V. Amine, K. et al., Journal of The Electrochemical Society 143, pp. 1607-1613 (1996), Strobel, P. et al., Journal of Material Chemistry 10, pp. 429-436 (2000), Morales, J. et al., Journal of Solid State Chemistry 2, pp. 420-426 (1998), and Wagemaker, M. et al., Journal of The American Chemical Society 143, pp. 13526-13533 (2004) describe the spinel LiNi0.5−xMn1.5+xO4 as 3V material. Specific capacities of up to 160 mAh/g during discharging from 3.5 V to 2.0 V after the first cycle were, however, not found to be stable after a number of cycles. Sun, Y.-K. et al, Journal of Power Sources 161 19-26, (2006) describes stable cycling of an LiNi0.5Mn1.5O4 spinel prepared by precipitation within a voltage range from 3.5 V to 2.4 V relative to Li/Li+ over 50 cycles, improved by doping with sulfur.
Electrochemical characterizations over the entire voltage range from 5.0 V to 2.0 V have only infrequently been carried out. They require additional lithium in the cell. Park et al., Electrochimica Acta 52, pp. 7226-7230 (2007) described the structural changes in the ordered and disordered spinel over the entire voltage range. The degradation of the 4V spinel LiMn2O4 over the voltage range from 5.0 V to 2.0 V is described in Johnson et al., Electrochemistry Communications 7, pp. 528-536 (2005). Glatthaar et al., 219th ECS Meeting Abstracts, Montréal, B1, 194 (2011) and Glatthaar et al., LiBD—Electrode Materials, Arcachon, O17 (2011) described results in respect of the deep discharging of iron- and fluorine-doped LiNi0.5Mn1.5O4, which display an increase in the cycling stability brought about by doping with iron and fluorine in the voltage range from 2.0 V to 5.0 V.